This specifies the number of wanted peers to the tracker and is
configurable from the commandline flag "--numwant".

Closes GH-5

btpd now uses all tiers in parallel, so a torrent with two tiers will
essentially be treated by btpd as a torrent with two trackers to send
each event to. This is not quite what the multitrackes standard says,
but it's much easier to implement reasonable behaviour this way.

btpd is going to report the number of good trackers for a torrent
instead of the number of errors, but for now just report zero errors.

Btpd can now use both ipv4 and ipv6. The new options -4 and -6 toggles
use of ip v4 and v6 respectively. They are both used by default.
Remove restrictions on the --ip option since the spec allows to be a
dns name. Ultimately this option may need to be changed on a per
tracker/torrent basis.

all btpd headers are included through btpd.h.

tracker reply.

allows me to add code for other types of trackers.

Remove the curl interface glue, since I use my own http client now. The curl
code was my main reason for using threads, so I'm a large step closer to make
btpd unthreaded again.